DIAGNOSIS The evaporator fan motor has likely failed. This fan moves cold air from the freezer into the refrigerator compartment. No air movement means the fridge section stays warm.
FIX Unplug the unit. Remove the interior back panel in the freezer compartment. You'll see a small fan mounted near the evaporator coils. Check if the fan blade spins freely by hand. If it's seized or doesn't spin when powered, the fan motor needs replacement. The part is typically listed as the evaporator fan motor or freezer fan motor.
IF THAT DIDN'T WORK The second suspect is a partially defrosted evaporator coil blocking airflow. If you see a solid block of ice on the coils behind that panel, the defrost system has failed. This is usually the defrost heater, defrost thermostat, or the main control board.
HEADS UP When testing, never bypass the defrost thermostat to apply power directly to the heater. You can melt plastic lines and cause a refrigerant leak.